1. Field of the Invention
The present invention relates to a liquid droplet ejection head, a liquid droplet ejection apparatus, and an image recording method, and more particularly, to a liquid droplet ejection head, a liquid droplet ejection apparatus and an image recording method in which a plurality of nozzles are arranged in a matrix configuration.
2. Description of the Related Art
In recent years, inkjet recording apparatuses have come to be used widely as data output apparatuses for outputting images, documents, or the like. In an inkjet recording apparatus, a desired image is formed on a recording medium by ejecting ink droplets from a plurality of nozzles in a print head (liquid droplet ejection head).
The print head used in an inkjet recording apparatus may be a full line head having one or more than one nozzle row of a length corresponding to the full width of the recording medium, or a serial head which forms dot rows in a main scanning direction by scanning a short head, which has a shorter length than the full width of the recording medium, in the breadthways direction of the recording medium (main scanning direction). A full line head is able to print onto the full area of the printable region of the recording medium by scanning the recording medium once, by moving the head and the recording medium relatively to each other in a direction substantially perpendicular to the breadthways direction of the recording medium (sub-scanning direction). Therefore, it is able to print at higher speed than a serial head.
A print head (matrix type head) is commonly known in which a plurality of nozzle are arranged in a matrix configuration (two-dimensionally) in order to achieve high quality in the image formed by the inkjet recording apparatus. For example, as shown in FIG. 30, there is a head in which a plurality of nozzles 51 are arranged in a matrix configuration, on the basis of a fixed arrangement pattern aligned in a row direction following the main scanning direction, which is perpendicular to the relative conveyance direction of the recording medium (the paper conveyance direction), and an oblique column direction which is not perpendicular to the main scanning direction. By constituting a print head of this kind as a full line head, it is possible to treat the nozzle rows when projected so as to align in the main scanning direction as a linear arrangement of nozzles, and it is possible to form a dot row of a single line in the main scanning direction of the recording medium, by driving the nozzles in a prescribed sequence, while moving the print head and the recording medium relatively with respect to each other. However, in the case of full line heads, there is a problem in that density non-uniformity tends to become more conspicuous in a prescribed direction, such as the main scanning direction on the recording medium, due to variation in the ejection characteristics, such as the volume and speed of flight of the ink droplets ejected from the nozzles. In particular, in the case of a matrix type head in which the nozzles are arranged in a matrix configuration, the spatial separation between the nozzles which are mutually adjacent in the main scanning direction is an additional factor which makes density non-uniformity become more conspicuous.
Therefore, technology has been proposed for reducing the visibility of the density non-uniformity which is liable to occur in a print head in which a plurality of nozzles are arranged in a matrix configuration in this fashion (namely, a matrix type head) (see Japanese Patent Application Publication Nos. 2004-90504 and 2004-167982).
Japanese Patent Application Publication No. 2004-90504 discloses a nozzle arrangement in which, in a dot row formed in the sub-scanning direction while moving a print head relatively in the main scanning direction, a dot of a different dot diameter is positioned between two mutually adjacent dots of the same dot diameter.
Japanese Patent Application Publication No. 2004-167982 discloses a nozzle arrangement in which the size of the dot diameter is varied in a dot row formed in the sub-scanning direction while moving the print head relatively in the main scanning direction.
In both Japanese Patent Application Publication Nos. 2004-90504 and 2004-167982, rather than increasing or decreasing the dot diameter in the dot row formed in the sub-scanning direction in a linear fashion, large and small dots are combined in the sub-scanning direction and hence the visibility of the density non-uniformity in the sub-scanning direction is reduced.
Problems of the following kinds occur in a matrix type head in the related art shown in FIG. 30.
In FIG. 30, P0 is taken to be the pitch of the nozzles in the main scanning direction, P1 is taken to be the pitch of the nozzles that are mutually adjacent in the main scanning direction (in other words, the pitch of the nozzles that eject droplets at the same timing), P2 is taken to be the pitch of the nozzles in the main scanning direction in the juncture region (the junction section between nozzle rows), P3 is taken to be the pitch of the nozzles in the sub-scanning direction (the paper feed direction), and P4 is taken to be the pitch of the nozzles in the sub-scanning direction in the juncture region (the junction section between nozzle rows).
The juncture region (nozzle row junction section) is the boundary (junction section) between one nozzle row extending in an oblique column direction and another nozzle row which is adjacent to same in the main scanning direction. Furthermore, a nozzle at the end of a nozzle row in the oblique column direction, in a juncture region, is called a “juncture region nozzle”. For example, there is a juncture region between the nozzle row 51A-1 constituted by the seven nozzles 51-11 to 51-17 which are aligned in the oblique column direction, and the nozzle row 51A-2 which is adjacent to the nozzle row 51A-1 in the main scanning direction, where the juncture region nozzles are nozzle 51-17 and nozzle 51-21. In the juncture regions, the nozzle pitch in the sub-scanning direction (in other words, the nozzle pitch in the sub-scanning direction between the juncture region nozzles) P4, is greater than the nozzle pitch P3 in the sub-scanning direction in the other regions.
In a matrix type head in the related art, if the head is accurately installed in such a manner that it forms a prescribed angle with respect to the conveyance direction of the recording paper (the paper freed direction), (for example, if the lengthwise direction of the head is perpendicular to the paper feed direction), then the nozzle pitch P2 in the main scanning direction in the juncture regions is equal to the nozzle pitch P0 in the main scanning direction in the other regions (in other words, P2=P0), and the nozzle row projected to the main scanning direction (projected nozzle row) has a uniform nozzle pitch of P0 (=P2). Therefore, as shown in the lower part of FIG. 30, a dot row is formed in which dots are arranged at regular intervals in the main scanning direction of the recording medium, at a dot pitch P that is equal to the nozzle pitch P0 (=P2).
However, if the print head is removed and reinstalled in a head maintenance operation, or the like, then a slight deviation may arise in the angle of the print head with respect to the paper feed direction. In cases of this kind, the nozzle pitch P2 in the main scanning direction in the juncture regions becomes different to the nozzle pitch P0 in the main scanning direction in the other regions (in other words, P2≠P0), and hence portions of high density and portions of low density appear in the dot row formed in the main scanning direction, and this may give rise to visible density non-uniformity in the main scanning direction.
For example, if the print head is installed in a state where it has been rotated in the direction of the arrow A1 in FIG. 30, then the nozzle pitch P0 in regions other than the juncture regions becomes slightly smaller, whereas the nozzle pitch P2 in the main scanning direction in the juncture regions becomes larger. Therefore the nozzle pitch P2 becomes greater than the nozzle pitch P0 in the main scanning direction in the other regions. Consequently, even in an ideal state where there is absolutely no error in the ejection volume or ejection direction of any of the nozzles, the dot row formed in the main scanning direction of the recording medium has a larger dot pitch in the portions corresponding to the juncture regions, and hence the density becomes lower in these portions. On the other hand, if the print head is installed in a state where it has been rotated in the direction of arrow A2 in FIG. 30, then conversely to the situation described in the previous example, the nozzle pitch P2 in the main scanning direction in the juncture regions becomes smaller than the nozzle pitch P0 in the main scanning direction in the other regions. Therefore, even if all of the nozzles are in an ideal state, a dot row formed in the main scanning direction on the recording paper has a smaller dot pitch in the portions corresponding to the juncture regions, and hence the density becomes higher in these portions. In this way, in the case of a matrix head, portions of different density are visible at the intervals of the nozzle pitch P1 between nozzles that are mutually adjacent in the main scanning direction.
In a matrix type head in the related art of this kind, since the nozzle pitch P4 in the sub-scanning direction in the juncture regions is greater than the nozzle pitch P3 in the sub-scanning direction in the other regions, and since the nozzle pitch P2 in the main scanning direction in the juncture regions changes conversely to the nozzle pitch P0 in the main scanning direction in the other regions when the head is rotated, then any slight deviation in the angle of the head with respect to the conveyance direction of the recording medium (the head angle) causes the nozzle pitch P2 in the main scanning direction in the juncture regions to greatly differ from the nozzle pitch P0 in the main scanning direction in the other regions. This gives rise to highly conspicuous density non-uniformity in the main scanning direction on the recording medium.
Japanese Patent Application Publication Nos. 2004-90504 and 2004-167982 do not take any account of density non-uniformity occurring in the juncture regions, and hence they cannot effectively reduce the visibility of density non-uniformity occurring in the juncture regions of a matrix type head of this kind.